One known process for producing a quartz glass comprises forming a porous quartz glass base by a vapor-phase reaction and heating this porous quartz glass base to vitrify it. With respect to the production of a porous quartz glass base to be used for forming, e.g., optical fibers, lenses, prisms, and other various optical materials, the process called the VAD method (vapor-phase axial deposition method) is widely known (see, for example, patent document 1).
In this VAD method, a raw material gas such as, e.g., silicon tetrachloride (SiCl4) and a fuel gas such as hydrogen and oxygen are supplied from a burner to a lower-end part of a rod-shape member made of a quartz glass (hereinafter referred to as a target) which is a starting member perpendicularly hung down in a reaction furnace. The raw material gas is hydrolyzed in an oxyhydrogen flame, and the fine silica particles (SiO2) thus yielded are adhered to and deposited on the lower-end part of the target to thereby form a porous quartz glass base. Subsequently, this porous quartz glass base is transferred to a heating furnace and sintered by heating with a heater to thereby obtain a transparent glass.
In producing a porous quartz glass base by the method described above, the following flame hydrolysis reaction occurs.2H2+O2→2H2O2H2O+SiCl4→SiO2+4HClAs a result, hydrogen chloride gas (HCl), which is unnecessary, is generated simultaneously with fine silica particles, which are necessary as a raw material for glass. Because of this, a technique for removal is being employed, such as, e.g., that shown in FIG. 3. In this technique, a cleaning column 102 is disposed besides the reaction furnace 101 for forming a porous quartz glass base 110 therein. The hydrogen chloride gas and other gases generated in the reaction furnace 101 are sent to the cleaning column 102 through a gas discharge pipe 103. In this cleaning column 102, the gases are sprinkled from above with water to yield hydrochloric acid 111. The acid is then discharged outside. This cleaning column 102 is equipped with a gas discharge pipe 104, a valve 105, and a gas discharge fan 106 for the purpose of regulating gas discharge amount, i.e., the amount of the gases including hydrogen chloride gas which come into the cleaning column 102 through the gas discharge pipe 103. By adjusting the degree of opening of the valve 105, the amount of the gases to be discharged is controlled.
Patent Document 1: JP-A-62-72536
However, in this apparatus for producing a porous quartz glass base, there are cases where fine silica particles come together with hydrogen chloride gas and other gases into the gas discharge pipe 103 which communicates the reaction furnace 101 with the cleaning column 102. These fine silica particles have a temperature as high as about 500° C. immediately after having entered the gas discharge pipe 103 from the reaction furnace 101. On the other hand, even the inner wall surface of the gas discharge pipe 103 is in a relatively low-temperature state because the outer wall surface of the gas discharge pipe 103 is frequently exposed to the surrounding atmosphere. Consequently, the fine silica particles which have entered the gas discharge pipe 103 are rapidly cooled and adhere to the inner wall surface of the gas discharge pipe 103 to narrow the passage. As a result, the flow rate of the gases including hydrogen chloride gas in the gas discharge pipe 103 changes and this in turn exerts adverse influences to cause, e.g., a decrease in the productivity of a porous quartz glass base. Because of these circumstances, it has been necessary to frequently clean the inside of the gas discharge pipe 103 in the course of the production of a porous quartz glass base. This cleaning operation has been troublesome.